Novel cyclopentadithiophene-based D-A copolymers for organic photovoltaic cell applications

Here, the synthesis, characterization, and photovoltaic properties of four new donor-acceptor copolymers are reported. These copolymers are based on 4,4-difluoro-cyclopenta[2,1-b:3,4-b′] dithiophene as an acceptor unit and various donor moieties: 4,4-dialkyl derivatives of 4H-cyclopenta[2,1-b:3,4- b′]dithiophene and its silicon analog, dithieno[3,2-b:2′,3′-d] -silol. These copolymers have an almost identical bandgap of 1.7 eV and have a HOMO energy level that varies from -5.34 to -5.73 eV. DSC and X-ray diffraction (XRD) investigations reveal that linear octyl substituents promote the formation of ordered layered structures, while branched 2-ethylhexyl substituents lead to amorphous materials. Polymer solar cells based on these copolymers as donor and PC₆₁BM as acceptor components yield a power conversion efficiency of 2.4%. A series of donor-acceptor copolymers based on cyclopentadithiophene and dithienosilol are synthesized via the Suzuki coupling reaction and used in polymer solar cells. The influence of silicon versus carbon atoms on the light harvesting properties of the copolymers is estimated and the phase behavior of the copolymers with linear and branched alkyl side groups is compared.